text: A simple notation may be used where little information on the mechanism for variability is either available or is not required to be conveyed:
  *~ (circa or approximately)
  Where there is a continuous range of composition this can be written e.g.,  for a mixture of  and  and  for a mixture of  and .
  The recommendation is to use the following generalised method e.g.
  *  for 
  *  for 
  Note that cation vacancies in  could be described by